Please Explain Solar Heat Gain (SHGC)

Maybe you have seen the dreaded acronym SHGC. Even people in the window business get confused with this Egyptian hieroglyphic. SHGC stands for Solar Heat Gain Coefficient, but what does that mean?

(For you old timers SC* =SHGC*0.87)*SC–Shading coefficient

Basically SHGC represents the ability of the insulated glass unit to resist or reflect the sun’s solar radiation. It is a number that is between 0 and 1. The lower the number the more solar radiation is resisted by the insulated glass unit. Clear as mud isn’t it. Unless you study it you really need real life examples to get your head wrapped around the concept. So in short you can use this to remember which way is up:

In Northern climates you want a high SHGC number. (That is, you want more solar radiation to penetrate the glass unit and heat the inside of the house.

In Southern climates you want a Low SHGC number. (You want less solar radiation to penetrate the glass unit and heat the inside of the house.

If you have time to think about it, it is simple. But in the course of everyday life we just do not have the time to think through this stuff. If you use this post as a future reference, just glance at the following:

Low SHGC=Low heat allowed in the house

High SHGC=Higher heat allowed in the house

In the world of insulated glass, we frequently talk about the “number of glass surface”. Let me explain that. In a typical “double pane” insulated unit (i.e., two pieces of glass sandwiched between a “spacer”) , there are four surfaces of glass that we are interested in defining. The first surface is always defined from the outside of the house. Thad’be surface #1. The second surface is on the other side of the outside piece of glass. (it’s on the inside of the insulated unit)–thad’be surface #2. The third surface is the next surface that a golf ball would hit on it’s way to the inside of the house. This is first side of the second piece of glass in the unit–thad’be the #3 surface. Anyone guess which surface #4 is….anyone…anyone….Bue-ller…?

That’s right the #4 surface is on inside of the house and is the surface that is the easiest to clean with Windex. Ok, so now you know how to define the glass surfaces. I’ll bet your life is complete now, huh. No really, now we can discuss the differences that exist as a result of which surface the Low-e glass is assigned. If you put the Low-e surface on the #2 surface, you will get a higher SHGC than if you put the Low-e on the #3 surface. Based on this if your located in the north, you would want the Low-e on the #2 surface and if your located in the south you’d want the Low-e assigned to the #3 surface.

But the funny thing is that the overall U-factor is not changed by the placement of the Low-e. This is a result of the fact that when the window is tested for heat loss, it is done in an indoor laboratory setting in which the sun’s rays never are introduced into the test scenario. If you study every manufacturer’s test reports, you will notice that the U-Factor is not changed as a result of which surface the Low-e is placed. I remember an old Looney Tunes cartoon in which Foghorn Leghorn* said: “It just don’t add up!” What gives with this…

Well, in the real world, it can make a difference, but that difference is not quantifiable by current test methods–the Sun’s electromagnetic radiation is excluded from the test method due to the variation induced by this factor. (There really is no way to simulate the sun’s electromagnetic radiation without a Federal budget of ga-zillions of dollars). The Sun’s energy fluctuates by the time of year because of the constantly changing angle of the Sun’s rays as a result of the orbital path of the Earth, add in solar flairs and other solar variables and it gets hairy, don’t it! Without a consistent control variable, it’s prudent to leave it out of the test method procedure.

Which leads to this statement:

At MGM we place the Low-e coating on the third surface as a standard. (We actually put the 6010 on the second surface and all other products on the the third. We tried to change but knew we would have inventory issues with customers.)